Optical elements including cholesteric layers having cholesteric regularity are extensively used as circular polarization controlling optical elements (circularly polarizing plates, color filters, etc.) for liquid crystal displays.
In order to obtain circularly polarizing plates capable of reflecting every visible light, or reflection-type color filters in which each pixel has different selective reflection wave ranges corresponding to red (R), green (G) and blue (B) colors, or optical elements that are used in transmission or semi-transmission liquid crystal displays to improve optical efficiency, it is necessary to broaden the selective reflection wave ranges of cholesteric layers, or vary the selective reflection wave ranges so that they respectively agree with the wave ranges of red, green and blue light. This necessity demands a method of controlling the selective reflection wage ranges of cholesteric layers (broadening or wavelength shift of the selective reflection wave ranges) with high accuracy and ease.
Heretofore, a method in which multiple cholesteric layers having selective reflection wave ranges centered at different wavelengths are laminated (Japanese Laid-Open Patent Publication No. 319235/1998) has been proposed as a method of obtaining a cholesteric layer having a broadened selective reflection wave range.
With this method, however, a broadened selective reflection wave range which is simply the sum of the selective reflection wave ranges of cholesteric layers laminated is obtained, and a continuous change in helical pitch can never be brought about in the laminate. It is therefore necessary to laminate a greater number of cholesteric layers to obtain a greatly broadened selective reflection wave range, and this makes the method less efficient. Further, cholesteric layers to be laminated have different refractive indexes, so that the loss of light reflection is caused at the interface between each two cholesteric layers laminated. Furthermore, since a number of cholesteric layers are laminated, it is not easy to make the resulting laminate thin. In addition, since this method requires a great number of steps to accomplish a full-color display, it has several disadvantages from the viewpoint of production efficiency and cost.
Another known method of obtaining a cholesteric layer having a broadened selective reflection wave range is that a liquid crystalline material capable of developing cholesteric structure with continuously varied helical pitches is used to form a cholesteric layer (U.S. Pat. No. 5,691,789, and Japanese Laid-Open Patent Publication No. 281814/1994).
This method does not require the lamination of multiple cholesteric layers, so that it can solve many of the aforementioned problems. However, the method is at a disadvantage in that since non-crosslinkable liquid crystalline molecules are used as the liquid crystalline material, the resulting cholesteric layer is poor in resistance to heat, and patterning, which is essential to accomplish a full-color display, cannot be conducted on such a cholesteric layer.
A further known method of obtaining a cholesteric layer having a broadened selective reflection wave range is that a liquid crystal having cholesteric regularity is brought into contact with a solvent or solvent mixture (Japanese Laid-Open Patent Publication No. 316755/1998).
However, the cholesteric layer obtained by this method has lowered color purity, so that a liquid crystal display in which such a cholesteric layer is incorporated cannot clearly display an image.
On the other hand, a method in which optically active groups constituting cholesteric structure are modified or deactivated (Japanese Laid-Open Patent Publication No. 54905/1998) has been proposed as a method of varying the selective reflection wave ranges of cholesteric layers so that they respectively agree with the wave ranges of light of various colors.
In this method, optically active groups are either modified or deactivated, so that the modified or deactivated molecules are to exist as impurities to impair the stability of the cholesteric layers. This method thus has the problem that a liquid crystal display containing these cholesteric layers cannot clearly display an image.